Conventional ion concentration sensors of FET type, such as an ion sensitive field effect transistor (ISFET) for detecting hydrogen ion concentration, have problems of low sensitivity, temporally variable outputs, and the like. In order to address such problems, for example, there has been proposed an FET ion sensor for detecting an amount of change in channel electric potential, which varies in accordance with change in surface electric potential of a sensing section (see Patent Literatures 1 through 4).
A surface of the above sensing section is typically covered with a silicon nitride film. Such a sensing section is configured to detect a hydrogen ion concentration through the use of change in surface electric potential of a semiconductor substrate, which change is caused by a reaction of (i) dangling bonds and (ii) hydrogen ions on a surface of the silicon nitride film. Each of FET ion sensors disclosed in respective Patent Literatures 1 through 4 is configured to output, as an analog signal, to external devices, an amount of change in electric potential which varies in accordance with hydrogen ion concentration.